The present invention generally relates to airfoils, and more particularly to coatings and methods for vibrational damping of fan and compressor airfoils of gas turbine engines.
Gas turbine engines employ airfoils that extend radially across the airflow path through the engine. Airfoils of rotating components of a gas turbine engine, usually termed blades (“buckets” in industrial turbines), extend radially outward from disks or rotors within the fan and compressor sections of the engines. During engine operation, the air flowing over these blades will vary in terms of speed, temperature, pressure, and density, resulting in the blades being excited in a number of different modes of vibration that induce bending and torsional twisting of the blades. The resulting vibration-induced stresses in the blades can cause high cycle fatigue (HCF), particularly if blades are excited at their resonant frequencies.
Several technologies have been investigated to address the need for damping fan and compressor airfoils. Notable examples include visco-elastic constraint layer damping systems (VE/CLDS), air-films, and internal dampers. U.S. Pat. No. 3,301,530 to Lull discloses a damping coating system that makes use of inner and outer metallic coatings, wherein the inner coating has a modulus of elasticity between that of the outer coating and the blade material. According to Lull, shear stresses generated by the differing moduli of elasticity are able to absorb energy and damp vibration in the blade. Another example of a damping coating system is disclosed in U.S. Pat. No. 3,758,233 to Cross et al., in which a ceramic material is employed as an outer coating that overlies an inner coating containing a mixture of the ceramic material and the blade material.
The existing damping technologies generally have limitations related to temperature, structural integrity, aerodynamic efficiencies, and manufacturing difficulties. While the use of thin damping coating systems are attractive from the standpoint of structural integrity, aerodynamic efficiencies, and manufacturing, the ability to survive at the operating temperatures of a gas turbine engine remains a challenge. Additional challenges associated with such coatings include spall resistance under thermal cycling conditions, erosion resistance, impact resistance, high temperature stability, and minimum impact on the fatigue lives of the airfoils. Finally, maximizing the damping effectiveness of the coating (lowest possible Q with the thinnest coating) is understood to be an important goal of any damping coating system.